Introduction

Air spaced etalons reduce the temperature dependence problem by using air as the etalon medium, to greatly reduce the change in  refractive index with temperature. The mirror spacing is now determined by spacers that may still be made from Fused Silica or by even more stable materials such as ULE or Zerodur.   

Air spaced etalons are significantly more complex since there are now three components involved, two end mirrors and the spacer. The outside surfaces of the end mirrors also need to be wedged and AR coated to avoid reflections from these surfaces causing additional unwanted etalon effects.

Introduction

Refer to our our standard solid fused silica etalons (we provide a lot of custom etalons so feel free to send us a request if these don't meet your needs or you require a thin film coating to enhance the finesse).  

Solid fused silica etalons are simple devices with a wide variety of applications in spectroscopy and lasers.  We have 3 styles of fused silica etalon;

• Uncoated
• Dielectric coatied
• Metal coasted

Introduction

It is very difficult to create very broadband dielectric coatings, however metal coatings are naturally effective over a wide range of wavelengths.  Sadly, they also absorb a significant amount of energy and as the reflectivity is pushed higher, the absorption also increases.  

Our metal coated etalons, or Metalons, are a compromise that achieves a reasonable finesse of about 2.5 using coatings that are about between 30% and 40% reflective from 350nm to 900nm.  The result is a peak transmission of about 40%, so if you need really high transmission and no loss then this is not your etalon.  

But the modulation is about 75% so if your application calls for wavelength monitoring then the metalon might just work great and will almost certainly work for your wavelength range!

Introduction

Germanium  etalons have a high index (~4.0 depending on wavelength) that creates a reasonably high finesse without any coatings.  They also remain transmissive farther out into the infrared than silicon etalons, the germanium etalon will still provide a decent signal at a wavelength of 14um. The high temperature sensitivity of germanium is similar to silicon and can also be useful (or problematic).  

These etalons are often used to monitor the wavelengths of tunable lasers (like lead salt or quantum cascade lasers) in the mid-infra red.  A portion of the output from your tunable laser is directed at the germanium etalon. As the laser is tuned, the transmission through the etalon is modulated with a spacing between transmission peaks equal to the etalon FSR.

Introduction

Solid silicon etalons are interesting because the high index of silicon (3.4 depending on wavelength) creates a reasonably high finesse without any coatings.

The temperature sensitivity of silicon can also be useful (or problematic). These etalons are often used to monitor the wavelengths of long wavelength lasers.

Introduction

There are a number of ways to tune etalons including:  tilting the entire etalon, moving the mirrors and changing the index of the medium (pressure, temperature, electrostatic).  

• Tilt tuning is a simple tuning technique. As the etalon is tilted the FSR changes with the cosine of the angle.   

• Piezo electric tuning of an air spaced etalon is usually what is meant by the phrase "tunable etalon".  This technique can tune the etalon quickly by changing the size of the air gap.  

Introduction

Solid YAG etalons are handy because the high index of YAG material (about 1.8 depending on wavelength). This creates a reasonably high finesse without any coating; this makes "yagalons" excellent for mode selection in high power solid state lasers. The additional advantage of YAG is excellent transmission in a wide range a wavelengths.

Introduction

Zinc Selenide etalons  have a reasonably high index (~2.5 depending on wavelength) that creates a reasonably high finesse without any coatings.

They also remain transmissive over a very broad wavelength range from 600nm to 20ums!.  

Thermal tuning is high but not as rapid as silicon or germanium.  Unfortunately zinc selenide is one of the few optical materials that we cannot fluid jet polish (due to the grain structure), so we cannot make the parallelism super uniform and coat them to create high finesse etalons.  But for low finesse (~1.6) etalons that work over a massive wavelength range, they are pretty handy!